Conventionally, a driver device having a motor and a control unit in one body is known. The control unit includes a rotation angle sensor for detecting a rotation angle of the motor and a semiconductor module having a switching element for switching the power supply to the winding of the stator of the motor. For example, in a patent document 1 (i.e., Japanese Patent Laid-Open No. 2012-152091), a semiconductor module is fixed to a heat sink, and the rotation angle sensor is attached to a substrate that is attached to the heat sink. The heat sink is disposed on an extension of an end part of a rotation shaft of the motor, and is fixed to the housing of the motor.
The heat sink and the substrate are made of respectively different materials. Therefore, the substrate suffers from a heat stress that is caused by the difference between thermal expansion coefficients of two different materials when the temperature environment changes. Here, in the patent document 1, since the shape of the heat sink is distorted due to the temperature change, the heat stress distribution concerning the substrate becomes uneven in a substrate plane. More specifically, there may be a big difference of heat stresses arising on both sides of the rotation angle sensor. Thus, the stress applied on the rotation angle sensor increases due to such a big difference of heat stresses, thereby, causing an increased detection error of the rotation angle sensor.